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class Solution(object):
    def combine(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: List[List[int]]
        """
        self.res = []
        self.cur = []
        self.backtracking(1,n,k)
        return self.res
    def backtracking(self, start,end, num):
        # To select `num` elements from `start`(including) to `end`(including).
        if (end-start)+1 < num:
            # If there's not enough numbers.
            return 
        if num == 0:
            # If `num` is used up, append self.cur to self.res
            self.res.append([each for each in self.cur])
            return
        for curNum in range(start, end+1):
            # For every curNum from start to end (both including),
            # append curNum to self.res and then backtracking from curNum+1 to end, for num-1 elements,
            # delete curNum after it is done.
            self.cur.append(curNum)
            self.backtracking(curNum+1,end,num-1)
            del self.cur[-1]